Modern industrial gas turbines are required to convert energy at a high efficiency while producing minimum polluting emissions. But these two requirements are at odds with each other since higher efficiencies are generally achieved by increasing overall gas temperature in the combustion chambers, while pollutants such as nitrogen oxide are typically reduced by lowering the maximum gas temperature. The maximum gas temperature can be reduced by maintaining a lean fuel-to-air ratio in the combustion chamber, but if the fuel/air mixture is too lean, incomplete fuel combustion can produce excessive carbon monoxide and unburned hydrocarbons. Other operational problems emerge when operating with lean combustion including unstable load transitions and combustion instability, also known as combustion dynamics. Therefore, the fuel/air mixture and the temperature in the reaction zone must be controlled to support complete combustion.
To balance the conflicting needs for increased efficiency and reduced emissions, extremely precise control is required to adjust the fuel/air mixture in the reaction zones of the combustors. Systems have been proposed for controlling the fuel/air mixture by monitoring various combustion parameters, and using the measured parameters as input to control the fuel system. For example, one conventional system includes a control system where fuel flow rates, pressure levels, and discharge exhaust temperature distributions are utilized as input for setting fuel trim control valves.
Other techniques for controlling combustion dynamics include measuring light emission from the combustion burner flame, and using the measured signal to control certain combustion parameters. For example, one conventional system uses a closed loop feedback system employing a silicon carbide photodiode to sense the combustion flame temperature via the measurement of ultraviolet radiation intensity. The sensed ultraviolet radiation is utilized to control the fuel/air ratio of the fuel mixture to keep the temperature of the flame below a predetermined level associated with a desired low level of nitrogen oxides.
Other conventional systems can use optical fibers for gathering and transmitting light from a combustion region to detectors. Yet other conventional systems can use a video camera to capture images of the flame primarily for monitoring the presence or absence of a flame.
Mass flux sensing techniques have been proposed for use in turbines. For example, laser-based Doppler-shift measurement systems may be used for determining air-flow in a turbine air inlet duct, and similar systems have been proposed for measuring the static temperature by comparing the absorption features from two light generators (lasers) of different frequency. A need remains for improved systems and methods for providing optical sensors.